In response to criticism from parents upset about the elimination of a traditional letter-based grading system for Billerica’s sixth-grade students, district officials are exploring the possibility of modifying the format of their report cards once again.

After receiving a petition signed by more than 600 people demanding the return of letter grades, the district is considering a hybrid model for the school system’s youngest middle school students. It would incorporate both the familiar percentage-based letter grades and standards-based assessment numbers, said Superintendent Timothy Piwowar.